1. Field of the Invention
This invention relates to a supported platinum alloy electrocatalyst, processes for its production, and to an electrode containing the catalyst for use in an acid-electrolyte fuel cell.
2. Description of the Prior Art
The fuel cell is an electrochemical device for directly converting a chemical energy generated from an oxidation-reduction reaction of a fuel such as hydrogen or hydrocarbons and an oxidizer such as oxygen gas supplied thereto into a low-voltage direct current. It is generally comprised of a fuel electrode (anode), an oxidizer electrode (cathode), an electrolyte interposed between the electrodes, and means for separately supplying a stream of the fuel and a stream of the oxidizer to the anode and the cathode, respectively.
An electrocatalyst is used in the anode and the cathode, and in operation, the fuel supplied to the anode is oxidized on the electrocatalyst in the presence of the electrolyte to release electrons. On the other hand, the oxidizing agent supplied to the cathode is reduced on the electrocatalyst in the presence of the electrolyte while consuming the electrons supplied from the anode via an external circuit. At this time, the current flowing through the external circuit is utilized as power under a fixed load.
Thus, the electrocatalyst plays an important role in the fuel cell, and the output and service life of the fuel cell depends greatly upon the activity of the electrocatalyst. In the early days, one or more noble metals selected from platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir), osmium (Os), silver (Ag) and gold (Au) which are among the elements of Groups 8, 9, 10 and 11 of the periodic table (according to the IUPAC nomenclature recommended in November 1983) were used in the form of metal black as the electrocatalyst.
Alloy catalysts comprising these noble metals in combination with one or more base metals such as aluminum, chromium, manganese, iron, cobalt, nickel and copper (U.S. Pat. Nos. 3,428,490 and 3,468,717), and skeleton catalysts resulting from dissolution of the base metal component from these alloys by acid or alkali treatment (U. S. Patent No. 3,429,750) have also been used.
However, because these catalysts have a low metal surface area, they require great quantities of noble metals. Furthermore, since they are very susceptible to sintering in the electrolyte, they have a short active lifetime and are not economical.
Later, a catalyst composed of a noble metal component dispersed and supported on a powdery carrier such as electrically conductive carbon black came into use. This greatly reduced the amount of the noble metal used and increased the economic advantage of fuel cell power generation. However, for a phosphoric acid fuel cell which is now most likely to become practical to gain widespread commercial acceptance as a power generation system, it requires an operation life of at least 40,000 hours at an output efficiency above a reasonable level. An oxygen/hydrogen type phosphoric acid fuel cell has the defect that the activation polarization of an oxygen reduction reaction at the cathode is by far greater than that of a hydrogen oxidation reaction at the anode, and moreover, in the presence of the electrolyte at high temperatures and molecular oxygen as an oxidizer, dissolving and sintering of the active metal readily proceed.
In recent years, in order to develop a fuel cell having a high efficiency, a long life and a low lost, investigations have been made on a carbon powder supported catalyst which is highly active mainly for an oxygen reduction reaction at the cathode. First, supported binary alloy catalysts composed of a platinumgroup metal (one of noble metals of groups 8, 9 and 10 of the periodic table) and a base metal of groups 2 to 6 of the periodic table such as vanadium, aluminum, titanium and chromium, which have a mass activity for oxygen reduction about twice that of a catalyst composed of platinum alone, were found (U. S. Patents Nos. 4186110, 4202934 and 4316944). Thereafter, for higher activity, supported platinum ternary alloy catalysts composed of platinum, vanadium and cobalt or platinum, chromium and cobalt (U.S. Pat. No. 4,447,506), a supported ternary alloy catalyst composed of platinum, cobalt and nickel (Japanese Laid-Open Patent Publication No. 8851/1986) and a supported ternary alloy catalyst composed of platinum, chromium and nickel (Japanese Laid-Open Patent Publication No. 319052/1988) were disclosed. On the other hand, a supported platinum-iron binary ordered alloy Pt.sub. 3 Fe "superlattice" (synonymous for "ordered") alloy) catalyst was disclosed (Japanese Laid-Open Patent Publication No. 7941/1985). Further, a ternary alloy comprising platinum, chromium and cobalt (U.S. Pat. No. 4,711,829) was again proposed as an ordered alloy catalyst.
The present inventors previously showed that a platinum-iron-cobalt ternary alloy catalyst (Japanese Laid-Open Patent Publication No 163746/1987) brings about an improvement not only in catalytic activity but also in the retention rate of the metal surface area. They also showed that a supported platinum-copper binary alloy catalyst (Japanese Laid-Open Patent Publication No. 269751/1987) surpasses conventional ordered and disordered multi-component alloys particularly in respect of the retention rate of the metal surface area. They also showed that a platinum-iron-copper Tulameenite-type Pt.sub.2 FeCu ternary tetragonal ordered alloy catalyst has improved catalytic activity and an improved service life (Japanese Patent Application No. 211621/1988).
However, none of these prior art catalysts can simultaneously satisfy activity and life required of practical fuel cells, and there is still room for improvement.